Deck it or not

[Schutzteacher]

This is my first build ever. So here’s the plan. I got a 99 XJ 4.0 engine in the machine shop, I sources a 258 Crank short snout out of a 80’s pacer. The head is already done and we’ve moved onto the block. I’m wanting to use the 4.0 connecting rods and KB438C pistons with a compression of 9.3:1. We are boring the cylinders to 30 over. I’ve already ordered the pistons.

In talking with the machine shop I’m wondering if the block needs to be decked down? Our main goal is to put the engine into an 83cj7 and use it as a fun cruising jeep. No mudding or rock crawling.

The machine shop thought it would be fine to level out the deck and then maybe take a little off the deck to tighten the quench but not enough to necessitate new push rods.

Th dish on the pistons is -21cc
The compression height is 1.353 on the pistons.

Any advice I can get would be appreciated

[Cheromaniac]

The SCR will be ~9.2:1 and the quench (assuming 0.043" head gasket) will be 0.070" if the block is left as it is. If you remove 0.020" from the deck, the SCR will increase to ~9.6:1 but the quench will decrease to 0.050".

[Schutzteacher]

So if I want to hit a .040 quench and have the shop deck it down .030 would that force me to need new custom push rods? I’m new at this and want to make sure that when I change one thing like deck height I can anticipate any other changes that may cause.

[Cheromaniac]

Yes you'd need shorter pushrods such as these:

https://www.summitracing.com/parts/cro- ... 1/overview